Add a block level usage flag for skip mode. If no block has chosen the
skip mode, the frame level flag for skip mode will be set off.

This patch also includes a small code cleanup, including the check on
whether the best RD mode is aligned with skip mode, if yes, the best
RD mode will be replaced by skip mode.

This patch slightly improves the coding performance of ext-skip.

Change-Id: If06092d5e32f15e63dcb5f35d32e68bc0f827c2b